Copper nano-ink, printed circuit board, and method for manufacturing printed circuit board substrate

Abstract

This copper nano-ink comprises copper nanoparticles and a solvent, wherein: the first average particle size of the copper nanoparticles is between 1 nm and 200 nm, the first average particle size being a volume-based average diameter obtained by analyzing a scanning electron microscope image of the copper nanoparticles; the copper nanoparticles include at least one of copper oxide and copper hydroxide. The copper nano-ink has a first ratio of 5.0% or less, which is the ratio of a mass ratio M2 to a mass ratio M1, the mass ratio M2 being the total mass ratio of copper oxide and copper hydroxide, and the mass ratio M1 being the mass ratio of copper, where the mass ratios are measured by the reference intensity ratio method.

Description

Copper nanoink, substrate for printed wiring board, and method for manufacturing substrate for printed wiring board

[0001] This disclosure relates to a copper nanoink, a substrate for a printed wiring board, and a method for manufacturing a substrate for a printed wiring board. This application claims priority to Japanese Application No. 2023-039679, filed March 14, 2023, and incorporates by reference all of the contents of said Japanese application.

[0002] In recent years, copper nanoink, in which copper nanoparticles are dispersed in a solvent such as water, has been used to form metal layers on printed wiring boards. The metal layers contain sintered bodies of copper nanoparticles. The metal layers are formed by applying the copper nanoink to the surface of a base film and then baking the resulting coating (Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2021-17641

[0004] The copper nanoink of the present disclosure comprises copper nanoparticles and a solvent, wherein the copper nanoparticles have a first average particle diameter of 1 nm or more and 200 nm or less, the first average particle diameter being a volume-based average diameter obtained by analyzing a scanning electron microscope image of the copper nanoparticles, the copper nanoparticles containing at least one of copper oxide and copper hydroxide, and a first ratio, which is the ratio of the total mass ratio M2 of copper oxide and copper hydroxide to the mass ratio M1 of copper measured by a reference intensity ratio method, in the copper nanoink, is 5.0% or less.

[0005] FIG. 1 is a schematic cross-sectional view showing a substrate for a printed wiring board according to an embodiment of the present disclosure.

[0006] [Problem to be Solved by the Present Disclosure] In the manufacturing process of copper nanoink, the copper nanoparticles contained in the copper nanoink are easily oxidized and turn into copper oxide (CuO, Cu ２ Furthermore, the reaction between the alkaline component used in the manufacturing process and the copper ions eluted from the copper nanoparticles can easily form copper hydroxide (Cu(OH) ２) is easily formed. As a result, the copper oxide and copper hydroxide concentrations in the copper nanoink tend to increase. When the copper oxide and copper hydroxide concentrations in the copper nanoink increase, the electrical conductivity of the sintered layer of copper nanoparticles obtained by firing the copper nanoink coating film tends to decrease. Therefore, there is a demand for copper nanoink that can produce a sintered layer of copper nanoparticles with high electrical conductivity.

[0007] The present disclosure aims to provide a copper nanoink capable of obtaining a sintered layer of copper nanoparticles having high electrical conductivity, a substrate for a printed wiring board having a sintered layer of copper nanoparticles having high electrical conductivity, and a method for manufacturing a substrate for a printed wiring board having a sintered layer of copper nanoparticles having high electrical conductivity.

[0008] [Effects of the Present Disclosure] According to the present disclosure, it is possible to provide a copper nanoink that can obtain a sintered body layer of copper nanoparticles having high conductivity, a substrate for a printed wiring board that includes a sintered body layer of copper nanoparticles having high conductivity, and a method for manufacturing a substrate for a printed wiring board that includes a sintered body layer of copper nanoparticles having high conductivity.

[0009] [Description of Embodiments of the Present Disclosure] First, embodiments of the present disclosure will be listed and described.

[0010] (1) The copper nanoink of the present disclosure comprises copper nanoparticles and a solvent, wherein the copper nanoparticles have a first average particle diameter of 1 nm or more and 200 nm or less, the first average particle diameter being a volume-based average diameter obtained by analyzing a scanning electron microscope image of the copper nanoparticles, the copper nanoparticles containing at least one of copper oxide and copper hydroxide, and a first ratio, which is the ratio of the total mass ratio M2 of copper oxide and copper hydroxide to the mass ratio M1 of copper measured by a reference intensity ratio method, is 5.0% or less.

[0011] According to the present disclosure, it is possible to provide a copper nanoink capable of obtaining a sintered layer of copper nanoparticles having high electrical conductivity, and a substrate for a printed wiring board including a sintered layer of copper nanoparticles having high electrical conductivity. "The first ratio is 5.0% or less" can also be expressed as "M2 / M1 is 0.05 or less."

[0012] (2) In (1) above, the second ratio, which is the ratio of the mass-based chlorine content C2 measured by ion chromatography to the mass-based copper content C1 measured by ICP optical emission spectroscopy in the copper nanoink, may be 0.3% or less.

[0013] According to this, the copper nanoparticles are less likely to aggregate. "The second ratio is 0.3% or less" can also be expressed as "C2 / C1 is 0.003 or less."

[0014] (3) In the above (1) or (2), the second average particle diameter of the copper nanoparticles may be 5 nm or more and 400 nm or less. The second average particle diameter is a number-based average diameter measured using an ultrasonic attenuation particle size distribution analyzer.

[0015] This can improve the dispersibility and stability of copper nanoparticles in the copper nanoink.

[0016] (4) In any one of (1) to (3) above, the first ratio may be 0.1% or more.

[0017] This makes it easier for a thin copper oxide layer and a thin copper hydroxide layer to exist around the copper nanoparticles, which tends to improve the affinity of the copper nanoparticles with the solvent or dispersant.

[0018] (5) The substrate for printed wiring board of the present disclosure includes a base film including a first main surface and a sintered layer of copper nanoparticles formed on the first main surface, and Cu of the sintered layer ２ The substrate for printed wiring boards has an O content of 3.0 mass % or less, and the gloss of the sintered body layer is 300 or more.

[0019] The substrate for printed wiring boards of the present disclosure can include a sintered layer of copper nanoparticles having high electrical conductivity.

[0020] (6) In the above (5), the resistivity of the sintered body layer may be 100 μΩ cm or less. This further improves the conductivity of the sintered body layer. When electroplating the sintered body layer, good electrical conductivity can be obtained if the resistivity of the sintered body layer is 100 μΩ cm or less.

[0021] (7) The method for manufacturing a substrate for a printed wiring board according to the present disclosure is the method for manufacturing a substrate for a printed wiring board according to (5) or (6) above, and includes the steps of applying the copper nanoink according to any one of (1) to (4) above to the base film, drying the coated film of the copper nanoink, and firing the dried film of the copper nanoink. In the firing step, the sintered body layer is formed on the base film.

[0022] This manufacturing method makes it possible to manufacture a substrate for a printed wiring board having a sintered layer of copper nanoparticles with high conductivity.

[0023] [Details of the Embodiments of the Present Disclosure] Specific examples of the copper nanoink and the substrate for printed wiring boards of the present disclosure are described below.

[0024] In the present disclosure, when one or more numerical values ​​are recited as the lower limit and the upper limit of a numerical range, a combination of any one numerical value recited as the lower limit and any one numerical value recited as the upper limit is also disclosed. For example, when a1, b1, and c1 are recited as the lower limit and a2, b2, and c2 are recited as the upper limit, the following are disclosed: a1 to a2, a1 to b2, a1 to c2, b1 to a2, b1 to b2, b1 to c2, c1 to a2, c1 to b2, and c1 to c2.

[0025] [Embodiment 1: Copper Nanoink] A copper nanoink according to one embodiment of the present disclosure (hereinafter also referred to as "Embodiment 1") comprises copper nanoparticles and a solvent. The copper nanoparticles have a first average particle diameter of 1 nm or more and 200 nm or less. The first average particle diameter is a volume-based average diameter obtained by analyzing a scanning electron microscope image of the copper nanoparticles. The copper nanoparticles include at least one of copper oxide and copper hydroxide. In the copper nanoink, a first ratio, which is the ratio of the total mass ratio M2 of copper oxide and copper hydroxide to the mass ratio M1 of copper measured by a reference intensity ratio method, is 5.0% or less.

[0026] In the copper nanoink, the first ratio is 5.0% or less, and the copper oxide and copper hydroxide contents in the copper nanoink are low. Therefore, by using this copper nanoink, a sintered layer of copper nanoparticles having high conductivity can be obtained.

[0027] <Copper Nanoparticles> <First Average Particle Diameter> The first average particle diameter of the copper nanoparticles is 1 nm or more and 200 nm or less. The lower limit of the first average particle diameter of the copper nanoparticles is 1 nm, or may be 10 nm or 20 nm. When the first average particle diameter of the copper nanoparticles is 1 nm or more, the dispersibility and stability of the copper nanoparticles in the copper nanoink can be improved. The upper limit of the first average particle diameter of the copper nanoparticles is 200 nm or more and may be 150 nm. When the first average particle diameter of the copper nanoparticles is 200 nm or less, the sintered body layer obtained by firing the copper nanoink is less likely to have an increased void, and the conductivity of the sintered body layer is improved. The first average particle diameter of the copper nanoparticles may be 10 nm or more and 150 nm or less, or may be 20 nm or more and 150 nm or less.

[0028] The first average particle diameter of the copper nanoparticles is a volume-based average diameter obtained by analyzing a scanning electron microscope image of the copper nanoparticles. The first average particle diameter corresponds to a volume-based average diameter based on the particle diameter of each copper nanoparticle. The method for measuring the first average particle diameter of the copper nanoparticles is as follows. A polyimide film is prepared, copper nanoink is coated on one side of the polyimide film, and the coated film is dried to form a dry film. The surface of the dry film is observed with a scanning electron microscope (SEM) (Hitachi High-Tech's "SU-8020" (trademark)) to obtain an SEM image. The observation magnification is 100,000 times. Two rectangular measurement fields of 900 nm x 1,300 nm are set in the SEM image. The two measurement fields are set so as not to overlap. Using image processing software ("Image-Pro" (trademark) manufactured by Hakuto Co., Ltd.), the equivalent circle diameters of all copper nanoparticles in the two measurement fields are measured, and the volume-based average diameter is calculated. "Equivalent circle diameter of copper nanoparticles in the measurement field" is the diameter of a circle having an area equal to the area of ​​the copper nanoparticles in the measurement field. The volume-based average diameter is also referred to as the volume mean diameter (MV) and is defined by the following equation 1: MV = Σ (Vi · di) / Σ (Vi) Equation 1 In equation 1, Vi represents the volume per particle, and di represents the particle diameter.

[0029] For copper nanoinks produced at the same time, it was confirmed that there was no significant difference between the measured values ​​even when multiple measurement fields were arbitrarily set.

[0030] <<First Ratio>> The copper nanoparticles contain at least one of copper oxide and copper hydroxide. In the copper nanoink, the first ratio, which is the ratio of the total mass ratio M2 of copper oxide and copper hydroxide to the mass ratio M1 of copper measured by the reference intensity ratio method, is 5.0% or less. The total mass ratio M2 of copper oxide and copper hydroxide is the sum of the mass ratio of copper oxide and the mass ratio of copper hydroxide. The copper oxide is CuO and Cu ２ O. Copper hydroxide is Cu(OH) ２ The copper oxides (CuO and Cu) contained in the copper nanoparticles ２It is presumed that copper hydroxide (Cu(OH)O) contained in the copper nanoparticles is formed by oxidation of the copper in the copper nanoparticles during the manufacturing process of the copper nanoink. ２ ) is thought to be formed when the copper in the copper nanoparticles reacts with the alkaline component used in the synthesis of the copper nanoink during the manufacturing process, and copper ions partially dissolved from the copper nanoparticles.

[0031] The upper limit of the first ratio is 5.0%, but may also be 4.5%, 4.0%, 3.5%, 3.0%, 2.4%, or 2.0%. When the first ratio is 5.0% or less, the copper oxide and copper hydroxide contents in the copper nanoink are low, making it possible to obtain a sintered layer of copper nanoparticles with high conductivity. The lower limit of the first ratio is not particularly limited. The first ratio may be 0% or more, more than 0%, or 0.1% or more. When the first ratio is 0.1% or more, a thin copper oxide layer or copper hydroxide layer is more likely to be present around the copper nanoparticles, which tends to improve the affinity of the copper nanoparticles with solvents and dispersants. The first ratio may be 0% or more and 5.0% or less, more than 0% and 5.0% or less, 0.1% or more and 5.0% or less, 0.1% or more and 4.0% or less, 0.1% or more and 3.0% or less, 0.1% or more and 2.4% or less, or 0.1% or more and 2.0% or less.

[0032] The presence of copper nanoparticles containing at least one of copper oxide and copper hydroxide can be confirmed by the following procedure. A polyimide film is prepared, and copper nanoink is coated on one side of the polyimide film. The coated film is dried in a vacuum to form a dry film. The copper nanoparticles contained in the dry film are subjected to a mapping analysis under vacuum using an energy dispersive X-ray spectrometer (EDX: Energy Dispersive X-ray Spectroscope, JEOL Ltd. "JED-2300T" (trademark)) (STEM-EDX) attached to a scanning transmission electron microscope (STEM: Scanning Transmission Electron Microscope, JEOL Ltd. "ARM200F" (trademark)). If at least one of copper oxide and copper hydroxide is confirmed at the periphery of the copper nanoparticles in the mapping image, the copper nanoparticles are determined to contain at least one of copper oxide and copper hydroxide.

[0033] The first ratio is obtained by obtaining an XRD spectrum of the copper nanoink by X-ray diffraction (XRD) and then using the reference intensity ratio (RIR) method based on the XRD spectrum. The conditions for the X-ray diffraction measurement are as follows: Measurement device: "SmartLab" manufactured by Rigaku Corporation X-ray used: Cu-Ka Excitation conditions: 45 kV, 200 mA Incident optical system: CBO-f Sample stage: XY stage Slit size: 0.8 mm Mask: 0.5 mm Receiving optical system: HyPix-3000 (two-dimensional) Scanning method: 2θ-θ scan Measurement range: 2θ is 30° to 80° Step width: 0.03° Scan speed: 2° / min

[0034] In the XRD spectrum of copper nanoink, Cu, Cu ２ O, CuO, and Cu(OH) ２ The integrated intensity of each of the strongest lines is calculated. The mass ratio of each component is calculated from this integrated intensity using the RIR values ​​registered in the database. In the XRD spectrum, the strongest line of Cu is present at a diffraction angle 2θ = 43.30°, and Cu ２The strongest O line is at a diffraction angle 2θ = 36.44°, the strongest CuO line is at a diffraction angle 2θ = 35.54° or 38.73°, and Cu(OH) ２ The strongest ray of is present at a diffraction angle 2θ=23.83°. The RIR value of each component is specified by referring to the powder X-ray diffraction database published by the International Center for Diffraction Data (ICDD).

[0035] The mass ratio of Cu corresponds to the mass ratio M1 of copper, and Cu ２ The mass ratio of O, the mass ratio of CuO and Cu(OH) ２ The sum of the mass ratios corresponds to the mass ratio M2 of the copper oxide and the copper hydroxide in total. The first ratio is calculated based on the mass ratio M1 of copper and the mass ratio M2 of the copper oxide and the copper hydroxide in total.

[0036] <Second Average Particle Diameter> The second average particle diameter of the copper nanoparticles may be 5 nm or more and 600 nm or less. The lower limit of the second average particle diameter of the copper nanoparticles may be 5 nm, 25 nm, or 80 nm. When the second average particle diameter of the copper nanoparticles is 5 nm or more, the dispersibility and stability of the copper nanoparticles in the copper nanoink can be improved. The upper limit of the second average particle diameter of the copper nanoparticles may be 600 nm, 400 nm, or 250 nm. When the second average particle diameter of the copper nanoparticles is 600 nm or less, particularly 400 nm or less, the sintered body layer obtained by firing the copper nanoink is less likely to have increased voids, improving the conductivity of the sintered body layer. The second average particle diameter of the copper nanoparticles may be 25 nm or more and 400 nm or less, or 80 nm or more and 250 nm or less.

[0037] The second average particle diameter of the copper nanoparticles is a number-based average diameter measured using an ultrasonic attenuation particle size distribution analyzer. The second average particle diameter corresponds to a number-based average diameter based on the equivalent sphere diameter of the copper nanoparticle aggregates in the copper nanoink. The "equivalent sphere diameter of the copper nanoparticle aggregates" refers to the diameter of a sphere equal to the volume of the copper nanoparticle aggregates. The ultrasonic attenuation particle size distribution analyzer "DT-100" (trademark) manufactured by Dispersion Technology can be used. The measurement range is 5 nm to 1000 μm. The number-based average diameter is also referred to as the number-average diameter (MN: Mean Number Diameter) and is defined by the following equation 2: MN = Σ(ni·di) / Σni Equation 2 In Equation 2, ni represents the number of particles, and di represents the particle diameter.

[0038] <Second Ratio> The copper nanoink may contain chlorine. The chlorine in the copper nanoink is produced by titanium trichloride or copper (II) chloride (CuCl ２ ) is presumed to be derived from the above. In the copper nanoink, the second ratio, which is the ratio of the mass-based chlorine content C2 (mass%) measured by ion chromatography to the mass-based copper content C1 (mass%) measured by ICP optical emission spectroscopy, may be 0.45% or less, 0.3% or less, 0.2% or less, 0.15% or less, 0.06% or less, or 0.02% or less. When the second ratio is 0.45% or less, and particularly 0.3% or less, the dispersibility and stability of copper nanoparticles in the copper nanoink can be improved. The lower limit of the second ratio is not particularly limited. The second ratio may be, for example, 0% or more, more than 0%, or 0.02% or more. The second ratio may be 0% or more and 0.45% or less, or 0.02% or more and 0.30% or less.

[0039] The second ratio is measured by the following procedure. The copper nanoink is diluted with water at a dilution ratio of 50,000 to obtain a diluted solution. The copper content (unit: mass%) of the diluted solution is measured by ICP optical emission spectroscopy. The measurement by ICP optical emission spectroscopy is performed using an "iCAP6300" (trademark) manufactured by Thermo Fisher Scientific. The chlorine content (unit: mass%) of the diluted solution is measured by ion chromatography. The measurement by ion chromatography is performed using an "ICS-2100" (trademark) ion chromatography system manufactured by Thermo Fisher Scientific. The second ratio is equal to {chlorine content (mass%) of the diluted solution / copper content (mass%) of the diluted solution}.

[0040] <Solvent> The solvent for the copper nanoink is not particularly limited. Water may be used as the solvent for the copper nanoink.

[0041] When water is used as the solvent, the water content in the copper nanoink may be 20 parts by mass or more and 1900 parts by mass or less per 100 parts by mass of copper nanoparticles. When the water content is 20 parts by mass or more, the copper nanoparticle content in the copper nanoink is not excessive, and the copper nanoink can be uniformly applied to the base film. When the water content is 1900 parts by mass or less, the copper nanoparticle content in the copper nanoink is sufficient, and a good sintered body layer having the required thickness and density can be formed on the surface of the base film of the printed wiring board substrate.

[0042] <Dispersant> The copper nanoink may contain a dispersant. Examples of dispersants include polyethyleneimine, polyvinylpyrrolidone, polyvinyl alcohol, and polyethylene glycol. When the copper nanoink contains a dispersant, the particles are less likely to aggregate and precipitate. These dispersants may be used alone or in combination of two or more. Hereinafter, polyethyleneimine, polyvinylpyrrolidone, polyvinyl alcohol, and polyethylene glycol will be collectively referred to as "dispersants."

[0043] In the copper nanoink, the ratio of the mass of the dispersant to the mass of copper may be 0.01% or more and 2.5% or less, or 0.1% or more and 1.5% or less. When two or more types of dispersants are used, the mass of the dispersants refers to the total mass of the dispersants.

[0044] <Additives> The copper nanoink may contain additives, such as ascorbic acid or amine polymers, as long as the additives do not impair the effects of the present disclosure.

[0045] <Method for Producing Copper Nanoink> A method for producing copper nanoink will now be described. The method for producing copper nanoink includes the steps of precipitating copper nanoparticles, washing the copper nanoparticles, and dispersing the copper nanoparticles in a solvent.

[0046] <<Step of Precipitating Copper Nanoparticles>> In the step of precipitating copper nanoparticles, copper nanoparticles are precipitated by a liquid-phase reduction method. In this step, copper ions are reduced in an aqueous solution containing a reducing agent, and copper nanoparticles are precipitated. The liquid-phase reduction method is, for example, a titanium redox method.

[0047] The step of precipitating copper nanoparticles includes a step of preparing an aqueous reducing agent solution and a step of reducing copper ions.

[0048] In the step of preparing a reducing agent solution, an aqueous solution containing a reducing agent that reduces copper ions is prepared. In the step of reducing copper ions, an aqueous solution containing copper ions is added to the reducing agent solution, or a water-soluble metal compound that generates copper ions upon ionization is added to the reducing agent solution.

[0049] Various reducing agents can be used in liquid-phase (aqueous solution) reaction systems that can reduce and precipitate copper ions. Examples of reducing agents include transition metal ions such as sodium borohydride, sodium hypophosphite, hydrazine, trivalent titanium ions (e.g., titanium trichloride) and divalent cobalt ions, ascorbic acid, reducing sugars such as glucose and fructose, and polyhydric alcohols such as ethylene glycol and glycerin. Trivalent titanium ions may also be used as the reducing agent. A liquid-phase reduction method using trivalent titanium ions as a reducing agent is the titanium redox method. In the titanium redox method, copper ions are reduced by the oxidation-reduction reaction when trivalent titanium ions are oxidized to tetravalent titanium ions, resulting in the precipitation of copper nanoparticles. The titanium redox method makes it easy to form copper nanoparticles with fine and uniform particle sizes.

[0050] The aqueous reducing agent solution may further contain, for example, a complexing agent, a dispersing agent such as polyvinyl alcohol, or a pH adjuster.

[0051] As the complexing agent, various conventionally known complexing agents may be used. Examples of the complexing agent include sodium citrate, sodium tartrate, sodium acetate, gluconic acid, sodium thiosulfate, ammonia, and ethylenediaminetetraacetic acid. One or more of these may be used. As the complexing agent, sodium citrate may be used.

[0052] The dispersant to be added to the aqueous reducing agent solution is, for example, polyethyleneimine, polyvinylpyrrolidone, or polyvinyl alcohol.

[0053] The pH adjuster to be added to the aqueous reducing agent solution is, for example, a common acid or alkali such as hydrochloric acid, sulfuric acid, nitric acid, sodium hydroxide, sodium carbonate, or ammonia. The pH of the aqueous reducing agent solution is, for example, 5 or more and 13 or less. If the pH of the aqueous reducing agent solution is too low, the metal precipitation rate slows and the particle size distribution tends to become broad. If the pH of the aqueous reducing agent solution is too high, the metal precipitation rate becomes excessive, and the precipitated copper nanoparticles may aggregate and form cluster-like or chain-like coarse particles.

[0054] In the copper ion reduction step, copper ions are introduced into an aqueous solution of a reducing agent, where the copper ions are reduced by the reducing agent, resulting in the precipitation of copper nanoparticles.

[0055] When a water-soluble copper compound is dissolved in water, copper ions are generated by ionization of the water-soluble copper compound. For example, the water-soluble copper compound is copper (II) nitrate trihydrate (Cu(NO ３ ) ２ ・3H ２ O), copper(II) sulfate pentahydrate (CuSO ４ ・5H ２ O), copper(II) chloride (CuCl ２ ), copper(II) sulfate pentahydrate (CuSO ４ ・5H ２ O).

[0056] If a water-soluble copper compound is directly added to the reducing agent solution, the reaction will proceed locally immediately after the compound is added. This may result in uneven particle size of the copper nanoparticles and a wide particle size distribution. Therefore, it is also possible to dissolve the water-soluble copper compound in water and add the resulting solution containing copper ions to the reducing agent solution.

[0057] The lower limit of the temperature of the aqueous reducing agent solution in the step of reducing copper ions may be 0°C or 15°C. When the temperature is 0°C or higher, good reduction reaction efficiency can be obtained. The upper limit of the temperature of the aqueous reducing agent solution in the step of precipitating may be 100°C, 60°C, or 50°C. When the temperature is 100°C or lower, the growth rate of copper nanoparticles becomes appropriate, and the particle size can be easily adjusted.

[0058] <Step of Washing Copper Nanoparticles> In the step of washing copper nanoparticles, the copper nanoparticles precipitated in the aqueous reducing agent solution are washed.

[0059] The reaction solution containing the precipitated copper nanoparticles is left for 12 hours or more to precipitate the copper nanoparticles. The supernatant is then removed by decantation. Approximately 95% of the supernatant is removed.

[0060] After removing the supernatant, the solution containing the copper nanoparticle precipitate is washed with an aqueous solution containing a dispersant. Specifically, an aqueous solution containing a dispersant is added to the solution containing the copper nanoparticle precipitate, and the mixture is stirred for 60 minutes using a stirrer to obtain a mixed solution. Hereinafter, the aqueous solution containing a dispersant is also referred to as a "dispersant aqueous solution." The same dispersant as used in the process of precipitating copper nanoparticles can be used. Examples of dispersants include polyethyleneimine, polyvinylpyrrolidone, polyvinyl alcohol, and polyethylene glycol. Next, the mixed solution after stirring is subjected to solid-liquid separation by centrifugation, and a dehydrated solid containing copper nanoparticles and a dispersant is recovered. Hereinafter, washing the solution containing the copper nanoparticle precipitate with an aqueous solution containing a dispersant is also referred to as "dispersant washing."

[0061] Pure water is added to the solid material and dispersed in a mixer, then centrifuged to recover the dehydrated solid material. Hereinafter, this process will also be referred to as "pure water washing." Pure water washing is performed again to recover the solid material containing copper nanoparticles.

[0062] By the step of washing the copper nanoparticles, ionic components that are not adsorbed to the copper nanoparticles can be removed.

[0063] The recovered solid material containing copper nanoparticles may be powdered by processes such as drying and crushing. In order to prevent aggregation, the solid material may not be powdered, but may be dispersed at a high concentration in an aqueous solution.

[0064] <<Step of Dispersing Copper Nanoparticles>> In the step of dispersing copper nanoparticles, a solid material containing copper nanoparticles is dispersed in a solvent so that the copper nanoparticles have an appropriate concentration. In this way, copper nanoink can be obtained.

[0065] The solvent for the copper nanoink may be water. Alternatively, a water-compatible organic solvent may be used as the solvent for the copper nanoink. Examples of the organic solvent include alcohols such as ethanol, IPA, ethylene glycol, and glycerin, and glycol ethers such as diethylene glycol monobutyl ether.

[0066] <Step of Adding Additive> The method for producing copper nanoink may further include a step of adding an additive such as ascorbic acid or an amine-based polymer to the dispersion of copper nanoparticles.

[0067] [Embodiment 2: Substrate for Printed Wiring Board] A substrate for printed wiring board according to one embodiment of the present disclosure (hereinafter also referred to as "Embodiment 2") will be described with reference to Fig. 1. The substrate for printed wiring board 1 comprises a base film 2 including a first main surface 4, and a sintered layer 3 of copper nanoparticles formed on the first main surface 4. The Cu of the sintered layer ２ The content of O is 3.0 mass % or less. The glossiness of the sintered body layer 3 is 300 or more.

[0068] The printed wiring board substrate 1 can have high electrical conductivity.

[0069] <Base Film> The base film 4 of the printed wiring board substrate 1 is insulating. The base film 4 is not particularly limited. A conventionally known base film may be used as the base film 4. The base film 4 is, for example, a polyimide film, a PET (polyethylene terephthalate) film, a PEEK (polyether ether ketone) film, or a fluororesin film.

[0070] <Sintered body layer> Cu of sintered body layer 3 ２ The content of O is 3.0 mass % or less. ２ The upper limit of the O content may be 2.0 mass % or 1.5 mass %. ２ The lower limit of the O content may be 0 mass %. ２ The O content may be 0% by mass or more and 3.0% by mass or less, 0% by mass or more and 2.0% by mass or less, or 0% by mass or more and 1.5% by mass or less.

[0071] Cu of sintered body layer 3 ２ The O concentration is measured by X-ray diffraction using "Empyrean" (trademark) manufactured by Malvern Panalytical.

[0072] The lower limit of the glossiness of the sintered body layer 3 is 300, and may be 310 or 320. When the glossiness of the sintered body layer 3 is 300 or higher, the conductivity of the sintered body layer 3 is improved. The upper limit of the glossiness of the sintered body layer 3 is not particularly limited, and may be, for example, 500. The glossiness of the sintered body layer 3 may be 300 or higher and 500 or lower, 310 or higher and 500 or lower, or 320 or higher and 500 or lower.

[0073] The glossiness of the sintered body layer 3 is measured using a Gloss Checker IG-410 (trademark) manufactured by Horiba Ltd.

[0074] The upper limit of the resistivity of the sintered body layer 3 may be 100 μΩ·cm, 90 μΩ·cm, 80 μΩ·cm, or 60 μΩ·cm. The lower limit of the resistivity of the sintered body layer 3 is not particularly limited, but may be, for example, 1.6 μΩ·cm. The resistivity of the sintered body layer 3 may be 1.6 μΩ·cm or more and 100 μΩ·cm or less, 1.6 μΩ·cm or more and 90 μΩ·cm or less, 1.6 μΩ·cm or more and 80 μΩ·cm or less, or 1.6 μΩ·cm or more and 60 μΩ·cm or less.

[0075] The resistivity of the sintered body layer 3 is measured using a "Loresta-GX MCP-T700" manufactured by Nitto Seiko Analytech Co., Ltd.

[0076] The average thickness of the sintered body layer 3 may be 0.01 μm or more and 5 μm or less, or 0.02 μm or more and 3 μm or less. The method for measuring the average thickness of the sintered body layer 3 is as follows. Using an X-ray fluorescence device (measuring device: "FT160S" (trademark) manufactured by Hitachi High-Technologies), the thickness is measured at five points on the sintered body layer 3. The average value of the thicknesses at the five points is the average thickness of the sintered body layer 3.

[0077] The substrate 1 for a printed wiring board can be used to manufacture a printed wiring board by a subtractive method or a semi-additive method. The printed wiring board manufactured using the substrate 1 for a printed wiring board has a conductive pattern including a layer obtained by patterning the sintered body layer 3.

[0078] <Method for manufacturing a substrate for printed wiring boards> A method for manufacturing a substrate 1 for printed wiring boards includes, for example, a step of applying the copper nanoink of embodiment 1 to a base film 4 (coating step), a step of drying the coated film of the copper nanoink (drying step), and a step of firing the dried film of the copper nanoink (firing step). In the firing step, a sintered body layer 3 is formed on the base film 4.

[0079] <Coating Step> In the coating step, the copper nanoink is applied onto the base film 2 to form a coating film.

[0080] The copper nanoink can be applied to the base film by a conventionally known coating method such as spin coating, spray coating, bar coating, die coating, slit coating, roll coating, dip coating, etc. For example, the copper nanoink may be applied to only a portion of the surface of the base film 4 by screen printing, a dispenser, or inkjet printing.

[0081] <Drying Step> In the drying step, the coated film of the copper nanoink is dried to form a dry film.

[0082] In the drying step, the coating film may be dried by heating or blowing air. The coating film may also be dried by blowing hot air onto the coating film. The temperature of the hot air may be such that the solvent in the copper nanoink does not boil. The temperature of the hot air is, for example, 15°C or higher and 80°C or lower. The speed of the hot air may be such that the coating film does not ruffle. The speed of the hot air at the surface of the coating film is, for example, 5 m / s or higher and 10 m / s or lower.

[0083] <<Baking Process>> In the baking process, the dried film is baked after the drying process. This evaporates or thermally decomposes the solvent and dispersant of the copper nanoink in the dried film, sintering the copper nanoparticles. In this way, a sintered layer 3 is formed on one surface (first main surface 4) of the base film 4. The baking process may be performed in nitrogen gas. If the copper nanoink contains polyethyleneimine, the polyethyleneimine has a reducing effect during the baking process, and the copper oxide in the copper nanoink is reduced to copper through a reduction reaction. In addition, the carbon that makes up the polyethyleneimine is oxidized and released as carbon dioxide. If the copper nanoink contains polyvinyl alcohol, the polyvinyl alcohol is resistant to oxidation, making the copper in the dried film less susceptible to oxidation.

[0084] The lower limit of the firing temperature in the firing step may be 300 ° C. or 325 ° C. If the firing temperature is 300 ° C. or higher, the copper oxide can be sufficiently reduced and the copper nanoparticles can be completely sintered. The upper limit of the firing temperature may be 390 ° C. or 375 ° C. If the firing temperature is 390 ° C. or lower, copper is less likely to oxidize.

[0085] The lower limit of the firing time in the firing step may be 10 minutes or 20 minutes. If the firing time is 10 minutes or more, the copper oxide can be sufficiently reduced and the copper nanoparticles can be completely sintered. The upper limit of the firing time may be 6 hours or 4 hours. If the firing time is 6 hours or less, copper is less likely to oxidize.

[0086] In the method for manufacturing the printed wiring board substrate 1, a metal may be laminated by electroless plating or electroplating on the sintered layer of copper nanoink formed after the firing step. This metal layer may be thicker than the sintered layer 4.

[0087] The present embodiment will be described in more detail with reference to examples, although the present embodiment is not limited to these examples.

[0088] [Test 1] In Test 1, the performance of the copper nanoink and printed wiring board substrate 1 of Embodiment 1 was evaluated against that of a copper nanoink and printed wiring board substrate produced by a conventional manufacturing method. <Production of Copper Nanoink> <Samples 1 to 7> <Step of Precipitating Copper Nanoparticles> Titanium trichloride as a reducing agent, sodium hydroxide as a pH adjuster, sodium citrate as a complexing agent, and a dispersant were dissolved in pure water to obtain a reducing agent aqueous solution. The type of dispersant used in each sample is shown in the "Dispersant Type" column in Table 1. The reducing agent aqueous solution was prepared in a 60 L reaction tank. Next, while stirring the reducing agent aqueous solution, copper nitrate trihydrate, which had been kept at the same temperature as the reducing agent aqueous solution, was added to the reducing agent aqueous solution to precipitate copper nanoparticles.

[0089] <<Process for Washing Copper Nanoparticles>> The post-reaction solution containing copper nanoparticles was left for 12 hours or more to precipitate the copper nanoparticles. Approximately 95% of the supernatant was then removed by decantation. An aqueous solution of pure water with a dispersant added (aqueous dispersant solution) was added to the solution containing the copper nanoparticle precipitate, and the mixture was stirred for 60 minutes using a stirrer to obtain a mixed solution. The type of dispersant used in each sample is shown in the "Dispersant Type" column in Table 1. The dispersant content of the aqueous dispersant solution was 30% by mass. The amount of aqueous dispersant solution added to the solution containing the copper nanoparticle precipitate was 300 g.

[0090] Next, the mixed solution after stirring was subjected to solid-liquid separation by centrifugation to recover a dehydrated solid containing copper nanoparticles and a dispersant (dispersant washing). The rotation speed of the centrifuge was 15,000 rpm.

[0091] 5 L of pure water was added to the solid matter and dispersed in a mixer, followed by centrifugation to recover the dehydrated solid matter (purified water washing). Purified water washing was performed again to recover the solid matter containing copper nanoparticles.

[0092] <<Step of Dispersing Copper Nanoparticles>> Next, pure water was added to the solid material containing copper nanoparticles to disperse them, thereby obtaining a copper nanoink.

[0093] <Samples 1-1 to 1-5> Copper nanoparticles were precipitated in the same manner as in Samples 1 to 7.

[0094] <<Step of Separating Copper Nanoparticles>> Next, the reducing agent aqueous solution containing the precipitated copper nanoparticles was centrifuged to separate it into a copper nanoparticle concentrate containing copper nanoparticles and a liquid phase using a centrifuge.

[0095] <<Step of Dispersing Copper Nanoparticles>> For Samples 1-2 to 1-4, a copper nanoparticle concentrate containing copper nanoparticles was filtered, and then pure water was added to obtain a copper nanoink.

[0096] For Samples 1-1 and 1-5, polyethyleneimine (PEI) and citric acid were added to the copper nanoparticle dispersion. The content of each component was then adjusted with pure water to obtain copper nanoinks having the compositions shown in the "Copper nanoink composition" column in Table 1.

[0097] For Sample 1-6, polyethyleneimine, polyvinyl alcohol, and citric acid were added to a dispersion of copper nanoparticles. The content of each component was adjusted with pure water to obtain a copper nanoink having the composition shown in the "Copper nanoink composition" column in Table 1. Sample 1-6 corresponds to Sample 18 in the examples of Patent Document 1.

[0098]

[0099] <Measurement of Copper Nanoink> The first average particle diameter of the copper nanoparticles, the second average particle diameter of the copper nanoparticles, the first ratio, and the second ratio were measured for the copper nanoink immediately after production. The measurement methods for each item were as described in embodiment 1. The results are shown in Table 2. For each sample, the component with the largest mass ratio among copper oxide and copper hydroxide is shown in the "Maximum Mass Ratio Component" column in Table 2. In all samples, the copper nanoparticles contained at least one of copper oxide and copper hydroxide.

[0100]

[0101] <Preparation of Substrate for Printed Wiring Board> <Coating Process and Drying Process> A polyimide film with an average thickness of 25 μm was used as the base film. Each sample copper nanoink was coated on one side of the polyimide film using the bar coating method. The coated film was dried with air at room temperature to form a dried film.

[0102] <<Firing Step>> Next, the dried film was fired in a hot air furnace filled with nitrogen gas at a firing temperature of 350°C for 30 minutes to form a sintered body layer with an average thickness of 0.2 µm. In this way, a substrate for printed wiring board of each sample was obtained.

[0103] <Measurement of Substrate for Printed Wiring Board> For the substrate for printed wiring board immediately after production, the glossiness of the sintered body layer, Cu content of the sintered body layer, ２ The O content and resistivity of the sintered body layer were measured. The measurement methods for each item were as described in embodiment 1. The results are shown in Table 2. When the resistivity of the sintered body layer is 100 μΩ cm or less, the resistivity of the sintered body layer is determined to be low, and a printed wiring board substrate including this sintered body layer is determined to have high conductivity.

[0104] <Discussion> The copper nanoinks and substrates for printed wiring boards of Samples 1 to 7 are Examples. The copper nanoinks and substrates for printed wiring boards of Samples 1-1 to 1-6 are Comparative Examples. The sintered body layers of the substrates for printed wiring boards of Samples 1 to 7 (Examples) have lower resistivities than the sintered body layers of the substrates for printed wiring boards of Samples 1-1 to 1-6 (Comparative Examples). Therefore, the substrates for printed wiring boards of Samples 1 to 7 (Examples) have higher conductivity than the substrates for printed wiring boards of Samples 1-1 to 1-6 (Comparative Examples).

[0105] The first average particle diameter of the copper nanoparticles in Sample 1 and Sample 1-2 is 70 nm. The second average particle diameter of the copper nanoparticles in Sample 1 is 80 nm, and the second average particle diameter of the copper nanoparticles in Sample 1-2 is 100 nm. Therefore, compared to Sample 1-2, the copper nanoparticles in Sample 1 are less likely to aggregate, and the dispersibility of the copper nanoparticles is improved. In addition, the first ratio and second ratio of Sample 1 are smaller than those of Sample 1-2.

[0106] The first average particle diameter of the copper nanoparticles in Sample 2 and Sample 1-3 is 20 nm. The second average particle diameter of the copper nanoparticles in Sample 2 is 25 nm, and the second average particle diameter of the copper nanoparticles in Sample 1-3 is 40 nm. Therefore, compared to Sample 1-3, the copper nanoparticles in Sample 2 are less likely to aggregate, and the degree of dispersion of the copper nanoparticles is improved. Furthermore, the first ratio and second ratio of Sample 2 are smaller than those of Sample 1-3.

[0107] The first average particle diameter of the copper nanoparticles in Sample 3 and Sample 1-4 is 150 nm. The second average particle diameter of the copper nanoparticles in Sample 3 is 175 nm, and the second average particle diameter of the copper nanoparticles in Sample 1-4 is 500 nm. Therefore, compared to Sample 1-4, the copper nanoparticles in Sample 3 are less likely to aggregate, and the dispersibility of the copper nanoparticles is improved. Furthermore, the first ratio and second ratio of Sample 3 are smaller than those of Sample 1-4.

[0108] Sample 1 was compared with Sample 1-2, Sample 2 was compared with Sample 1-3, and Sample 3 was compared with Sample 1-4. From these comparison results, it is presumed that the process of washing the copper nanoparticles contributes to the resistance of the copper nanoparticles to aggregation, improvement of the dispersion of the copper nanoparticles, reduction of the first ratio, and reduction of the second ratio.

[0109] [Test 2] In Test 2, the copper nanoink of Sample 1 was sealed and stored for three months, and then the first average particle diameter, second average particle diameter, first ratio, and second ratio of this copper nanoink were measured. A substrate for a printed wiring board was produced using this copper nanoink in the same manner as Sample 1, and the gloss and resistivity of the sintered body layer were measured. The results are shown in Table 3. In Table 3, the copper nanoink of "Sample 2-1" is the copper nanoink of Sample 1 that had been sealed and stored for three months. The sintered body layer of "Sample 2-1" is a sintered body layer formed using the copper nanoink of Sample 1 that had been sealed and stored for three months.

[0110]

[0111] <Discussion> After the copper nanoink of Sample 1 was sealed and stored for three months (copper nanoink of Sample 2-1), aggregation and oxidation progressed slightly. However, the resistivity of the sintered body layer of the printed wiring board substrate made using the copper nanoink after three months of storage was low, and the printed wiring board substrate had high conductivity.

[0112] Although the embodiments and examples of the present disclosure have been described, it is intended from the beginning that the configurations of the above-described embodiments and examples may be appropriately combined and modified in various ways. The embodiments and examples disclosed herein are illustrative in all respects and should not be considered limiting. The scope of the present invention is defined not by the above-described embodiments and examples but by the claims, and is intended to include meanings equivalent to the claims and all modifications within the scope of the claims.

[0113] REFERENCE SIGNS LIST 1 Substrate for printed wiring board 2 Base film 3 Sintered body layer 4 First main surface

Claims

1. A copper nanoink comprising copper nanoparticles and a solvent, The first average particle diameter of the copper nanoparticles is 1 nm or more and 200 nm or less, The first average particle diameter is a volume-based average diameter obtained by analyzing a scanning electron microscope image of the copper nanoparticles, The copper nanoparticles contain at least one of copper oxide and copper hydroxide, In the copper nanoink, a first ratio, which is the ratio of the mass ratio M2 of the sum of the copper oxide and the copper hydroxide to the mass ratio M1 of copper measured by a reference intensity ratio method, is 5.0% or less.

2. The copper nanoink described in claim 1, wherein in the copper nanoink, a second ratio, which is the ratio of the mass-based chlorine content C2 measured by ion chromatography to the mass-based copper content C1 measured by ICP optical emission spectroscopy, is 0.3% or less.

3. The second average particle diameter of the copper nanoparticles is 5 nm or more and 400 nm or less, The copper nanoink according to claim 1 or 2, wherein the second average particle size is a number-based average particle size measured using an ultrasonic attenuation particle size distribution analyzer.

4. The copper nanoink according to claim 1 or claim 2, wherein the first ratio is 0.1% or more.

5. a base film including a first major surface; A sintered layer of copper nanoparticles formed on the first main surface, Cu of the sintered body layer ２ The content of O is 3.0 mass% or less, The sintered body layer has a glossiness of 300 or more.

6. 6. The substrate for printed wiring boards according to claim 5, wherein the resistivity of the sintered body layer is 100 μΩ·cm or less.

7. A base film including a first main surface; A sintered layer of copper nanoparticles formed on the first main surface, the sintered body layer has a Cu 2 O content of 3.0 mass% or less; A method for producing a substrate for a printed wiring board, wherein the glossiness of the sintered body layer is 300 or more, A step of applying the copper nanoink according to claim 1 or claim 2 to the base film; drying the coated film of the copper nanoink to form a dried film of the copper nanoink; Firing the dried film of the copper nanoink; Equipped with In the firing step, the sintered body layer is formed on the base film.